Flying Capacitor (C1)
The value of the flying capacitor (C1) affects the load
regulation and output resistance of the charge pump. A
C1 value that is too small degrades the device’s ability to
provide sufficient current drive, which leads to a loss of
output voltage. Increasing the value of C1 improves load
regulation and reduces the charge-pump output resis-
tance to an extent. See the Output Power vs. Charge-
Pump Capacitance graph in the Typical Operating
Characteristics. Above 2.2F, the on-resistance of the
switches and the ESR of C1 and C2 dominate.
Output Capacitor (C2)
The output capacitor value and ESR directly affect the
ripple at CPVSS. Increasing the value of C2 reduces
output ripple. Likewise, decreasing the ESR of C2
reduces both ripple and output resistance. Lower
capacitance values can be used in systems with low
maximum output power levels. See the Output Power
vs. Charge-Pump Capacitance graph in the Typical
Operating Characteristics.
CPVDD Bypass Capacitor
The CPVDD bypass capacitor (C3) lowers the output
impedance of the power supply and reduces the impact
of the MAX9756/MAX9757/MAX9758’s charge-pump
switching transients. Bypass CPVDD with C3, the same
value as C1, and place it physically close to CPVDD and
PGND (refer to the MAX9756/MAX9757/MAX9758
Evaluation Kit for a suggested layout).
Powering Other Circuits
from a Negative Supply
An additional benefit of the MAX9756/MAX9757/
MAX9758 is the internally generated negative supply
voltage (CPVSS). CPVSS is used by the MAX9756/
MAX9757/MAX9758 to provide the negative supply for
the headphone amplifiers. It can also be used to power
other devices within a design. Current draw from
CPVSS should be limited to 5mA; exceeding this affects
the operation of the headphone amplifier. A typical
application is a negative supply to adjust the contrast
of LCD modules.
When considering the use of CPVSS in this manner,
note that the charge-pump voltage of CPVSS is roughly
proportional to CPVDD and is not a regulated voltage.
Layout and Grounding
Proper layout and grounding are essential for optimum
performance. Use large traces for the power-supply
inputs and amplifier outputs to minimize losses due to
parasitic trace resistance, as well as route heat away
from the device. Good grounding improves audio per-
formance, minimizes crosstalk between channels, and
prevents any switching noise from coupling into the
audio signal. Connect CPGND, PGND, and GND
together at a single point on the PC board. Route
CPGND and all traces that carry switching transients
away from GND, PGND, and the traces and compo-
nents in the audio signal path.
Connect all components associated with the charge
pump (C2 and C3) to the CPGND plane. Connect VSS
and CPVSS together at the device. Place the charge-
pump capacitors (C1, C2, and C3) as close to the
device as possible. Bypass HPVDD and PVDD with a
1F capacitor to GND. Place the bypass capacitors as
close to the device as possible.
Use large, low-resistance output traces. As load imped-
ance decreases, the current drawn from the device out-
puts increase. At higher current, the resistance of the
output traces decrease the power delivered to the load.
For example, when compared to a 0
trace, a 100m
trace reduces the power delivered to a 4
load from
2.1W to 2W. Large output, supply, and GND traces also
improve the power dissipation of the device. The
MAX9756/MAX9757/MAX9758 thin QFN package fea-
tures an exposed thermal pad on its underside. This pad
lowers the package’s thermal resistance by providing a
direct-heat conduction path from the die to the PC
board. Connect the exposed thermal pad to GND by
using a large pad and multiple vias to the GND plane.
MAX9756/MAX9757/MAX9758
2.3W Stereo Speaker Amplifiers and DirectDrive
Headphone Amplifiers with Automatic Level Control
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